mm/page_ext: move page_ext_operations definition under CONFIG_PAGE_EXTENSION

page_ext_operations should only be defined when CONFIG_PAGE_EXTENSION is
enabled.

Besides, this may detect missing reliance on CONFIG_PAGE_EXTENSION from
future Page Extension clients at compile time.

Link: https://lkml.kernel.org/r/20230717113227.1897173-4-shikemeng@huaweicloud.comSigned-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/page_ext.h

@@ -8,6 +8,7 @@
 
 struct pglist_data;
 
+#ifdef CONFIG_PAGE_EXTENSION
 /**
  * struct page_ext_operations - per page_ext client operations
  * @offset: Offset to the client's data within page_ext. Offset is returned to
@@ -29,8 +30,6 @@ struct page_ext_operations {
 	bool need_shared_flags;
 };
 
-#ifdef CONFIG_PAGE_EXTENSION
-
 /*
  * The page_ext_flags users must set need_shared_flags to true.
  */